The present invention is a method of accurately and non-invasively predicting the temperature distribution in a volume of thermally treated tissue.
The prostate gland is a complex, chestnut-shaped organ which encircles the urethra immediately below the bladder. Nearly one third of the prostate tissue anterior to the urethra consists of fibromuscular tissue that is anatomically and functionally related to the urethra and the bladder. The remaining two thirds of the prostate is generally posterior to the urethra and is comprised of glandular tissue. The portion of the urethra extending through the prostate (i.e., the prostatic urethra) includes a proximal segment, which communicates with the bladder, and a distal segrnent, which extends at an angle relative to the proximal segment by the verumontanum.
Although a relatively small organ, the prostate is the most frequently diseased of all internal organs and is often the site of a common affliction among older men, benign prostatic hyperplasia (BPH), as well as a more serious affliction, cancer. BPH is a nonmalignant, bilateral expansion of prostate tissue occurring mainly in the transition zone of the prostate adjacent to the proximal segment of the prostatic urethra. As this tissue grows in volume, it encroaches on the urethra extending into the region of the bladder neck at the base of the bladder. Left untreated, BPH causes obstruction of the urethra which usually results in increased urinary frequency, urgency, incontinence, nocturia and slow or interrupted urinary stream. BPH may also result in more severe complications, such as urinary tract infection, acute urinary retention, hydronephrosis and uraemia.
Benign prostatic hyperplasia (BPH) may be treated using transurethral thermal therapy as described in U.S. Pat. No. 5,620,480 entitled METHOD FOR TREATING BENIGN PROSTATIC HYPERPLASIA WITH THERMAL THERAPY and in U.S. Pat. No. 5,575,811 entitled BENIGN PROSTATIC HYPERPLASIA TREATMENT CATHETER WITH URETHRAL COOLING, both of which are hereby incorporated by reference. During transurethral thermal therapy, the transition zone of the prostate is heated to necrose the tumorous tissue that encroaches on the urethra. Transurethral thermal therapy is administered by use of a microwave antenna-containing catheter which includes a multi-lumen shaft. The catheter is positioned in the urethra with the microwave antenna located adjacent to the hyperplastic prostatic tissue. Energization of the microwave antenna causes the antenna to emit electromagnetic energy which heats tissue within the prostate. A cooling fluid is circulated through the catheter to preserve tissue such as the urethral wall between the microwave antenna and the target tissue of the prostate. A rectal probe carrying a temperature sensor is often utilized as well, to ensure that the rectum is not overheated by the thermal therapy procedure.
In the process of performing thermal therapy of tissue as described in the above-referenced patents, it is desirable to ascertain the temperatures achieved in the tissue volume. Destruction of cells, or necrosis, in the targeted tissue volume is an objective of the thermal therapy. By monitoring the tissue temperature, necrosis may be determined according to the time/temperature relationship governing the response of the cells to heat levels above about 45.degree. C. As a result, once a predetermined volume of necrosis has been achieved to relieve the patient from the symptoms of BPH, the thermal therapy session may be discontinued, thereby minimizing the overall therapy time. This is an important objective in a thermal therapy treatment, so that the patient receives an optimal dosage of thermal treatment and experiences a minimal level of discomfort and intimidation prior to and during the procedure, and also to allow a greater number of procedures to be performed in a fixed amount of time.
There are several practical obstacles to accurately measuring or modeling the temperatures achieved in the targeted tissue volume being treated. Interstitially introducing temperature sensors into the targeted tissue volume is a highly invasive procedure that requires piercing of the tissue, and tends to negate the advantages of delivering microwave energy from an adjacent body lumen or cavity. However, efforts to deduce the temperatures achieved in the targeted tissue volume are typically subject to a rather considerable error oruncertainty, since there are a number of parameters that vary rather significantly from patient to patient and therefore tend to inhibit accurate tissue temperature prediction. For example, the blood flow rate, or perfusion, in the tissue volume being treated varies among patients, and even changes throughout the procedure as cells in the tissue volume are heated and necrosed. The size of the tissue volume may also vary among patients, causing difficulties if a certain percentage of necrosis in the tissue volume is desired. Therefore, there is a continuing need in the art for an improved system for accurately and non-invasively predicting the temperature distribution achieved in a volume of thermally treated tissue.